5.12 Cooling Systems
The chilled water systems shall have a 10°C (50°F) temperature differential in the central system, at the central plant, with a design supply water temperature between 4°C and 7°C (40°F and 45°F). In climates with low relative humidity, an 8°C (46°F) may be used. The chilled water system shall have a 6°C (43°F) temperature differential in the secondary systems, at the terminal points of use, such as coils with a design supply water temperature between 4°C and 7°C (40°F and 45°F).
District chilled water, if available, shall be used for cooling only if determined to be economical and reliable through a life cycle cost analysis.
Mechanical equipment rooms must be designed in accordance with the requirements of ASHRAE Standard 15: Safety Code for Mechanical Refrigeration. Chiller leak detection and remote alarming shall be connected to the BAS.
Freeze Protection. Propylene glycol manufactured specifically for HVAC Systems is used for freeze protection, primarily in low temperature chilled water systems (less than 4°C) (less than 40°F). The concentration of antifreeze should be kept to a practical minimum because of its adverse effect on heat exchange efficiency and pump life. The water make-up line for glycol systems shall be provided with an in-line water meter to monitor and maintain the proper percentage of glycol in the system. All coils exposed to outside airflow (at some time) shall be provided with freeze protection thermostats and control cycles. Provisions shall be made for drain down, storage and re-injection of the glycol into the system.
Condenser Water. All water-cooled condensers must be connected to a recirculating heat-rejecting loop. The heat rejection loop system shall be designed for a 6°C (43°F) temperature differential and a minimum of 4°C (40°F) wet bulb approach between the outside air temperature and the temperature of the water leaving the heat rejection equipment. Heat tracing shall be provided for piping exposed to weather and for piping down to 3 feet below grade.
Water Treatment. See section: Humidification and Water Treatment of this chapter for water treatment.
Special Cooling SystemsWaterside Economizer Cycle. In certain climate conditions cooling towers are capable of producing condenser water cold enough to cool the chilled water system without chiller operation. This option shall be considered in life cycle cost comparisons of water cooled chillers.Waterside economizer cycles are particularly cost effective in the low humidity climates of the western United States. In the eastern United States, enthalpy airside economizer cycles tend to produce lower operating costs. However, where used, any airside economizer shall be set so that no air with a dew point above 10°C (50°F) is allowed into the building.Waterside economizer systems shall be used only in areas where the outside air temperature will be below 4.4°C (40°F) wet bulb. Waterside economizers shall utilize a plate heat exchanger piped in parallel arrangement with its respective chiller. See “Air Distribution Systems, Air-Handling Units, and Airside Economizer Cycle” of this chapter.
Computer Room Air-Conditioning Units. Mainframe computer rooms shall be cooled by self-contained units for loads up to 280 kW (80 tons). These units shall be specifically designed for this purpose and contain compressors, filters, humidifiers and controls. They shall be sized to allow for a minimum of 50 percent redundancy, either two units at 75 percent load or three units at 50 percent. If the nature of the computer room is critical (as determined by consulting the GSA’s Office of the Chief Information Officer), three units sized at 50 percent of the design load shall be used. Heat rejection from these self-contained units shall be by air-cooled condensers or recirculating water-cooled condensers connected to a cooling tower or evaporative-cooled condenser.Water-side free cooling shall be utilized when possible.
For cooling loads greater than 280 kW (80 tons), chilled water air-handling systems shall be considered in a life cycle cost analysis. A dedicated chiller(s) is preferred, unless other parts of the building also require 24-hour cooling. The 24-hour cooling needs of a computer room should be identified in the HVAC, HVAC System Components, Sizing and Selection Standards for Equipment and Systems section of this chapter. The dedicated chiller plant shall provide some means of redundant backup, either by multiple machines or connection to the facility’s larger chilled water plant.
In large computer installations (areas of 500 m2(5,000 ft2)) it is recommended to segregate cooling of the sensible load (computer load) and control of the outside air ventilation and space relative humidity by using two separate air-handling systems. In this design, one unit recirculates and cools room air without dehumidification capability. This unit is regulated by a room thermostat. The second unit handles the outside air load, provides the required number of air changes and humidifies/ dehumidifies in response to a humidistat. This scheme avoids the common problem of simultaneously humidifying and dehumidifying the air.
For ventilation, air-handling, and humidification requirements of computer rooms, see sections Air Distribution Systems, Air-Handling Units, Computer Room Air-Handling of this chapter. The room temperature conditions shown in Table 5-1 provide a higher available temperature for reduced fan power consumption and easier winter humidification. Verify with users to determine if the air- conditioning system must be connected to emergency power system. These systems should be provided with an alternative power source, connected to emergency generators, if the computer room houses critical components. Consult GSA’s Office of the Chief Information Officer to determine which computer rooms meet this requirement.
Desiccant Cooling. For high occupancy applications where moisture removal is required, solid desiccant with silica gel may be used in combination with mechanical cooling. Heat recovery wheels may be used prior to the mechanical cooling process. Desiccant cooling units shall be equipped with airflow-setting devices for both process and reactivation air flows, and shall be equipped with gauges or digital displays to report those air flows continuously. The desiccant cooling system shall have self-contained microprocessor controls capable of connecting to and interoperating with a direct digital control (DDC) Building Automation system. Natural gas or condenser waste heat shall be used as fuel for reactivation of the desiccant. Lithium chloride liquid desiccants are not permitted.